The invention relates to a conventional steam-cracking unit that comprises at least one furnace with pyrolysis pipes that are connected downstream to at least one multitube-type quenching exchanger for cracked gases with a tubular input plate. Such conventional units use means for introducing decoking agents that consist of lines for introducing decoking fluids from the group of air, vapor, and air/vapor mixtures.
It is known to one skilled in the art that these conventional steam-cracking furnaces are subject to erosion phenomena due to the circulation of deslagged coke fragments from the pyrolysis pipes during decoking periods (with air, with vapor, or with air/vapor mixtures). Said coke fragments are endogenous coke fragments (inside the unit) and give rise to serious erosion phenomena, whereas within the framework of this invention only erosive (external) solid particles are introduced into the unit.
The prior art is illustrated by Patent Applications WO 90 12851 and EP-A-036151. In addition, Application WO 96 20259 describes a decoking process for steam-cracking furnace pipes by erosive particles that are introduced upstream from a distributor impact separator at the input of an indirect quenching exchanger.
It is also known to one skilled in the art that the most serious erosion problems are observed at the tubular plate of the quenching exchanger, which is very generally thin (about 10 mm) and therefore fragile.
The object of the invention is to propose a unit that uses means of protection against erosion, in particular at the tubular input plate of the exchanger.
Another object of the invention is to propose means of protection that do not pose obstruction risks during times when there is circulation of considerable amounts of deslagged coke fragments from the upstream walls (pyrolysis pipes).
Finally, another object of the invention is to propose holding means that are strong and reliable for the protection means inside a cone with a small internal volume.
For this purpose, a hydrocarbon steam-cracking unit in pyrolysis pipes that comprises at least one furnace that comprises at least one multitube quenching exchanger with a tubular input plate is proposed, wherein this unit uses means for introducing decoking agents and wherein said means consist of a number of lines for introducing decoking fluids from the group of air, water vapor, and air/vapor mixtures, wherein said quenching exchanger comprises an input cone that is connected upstream by a transfer pipe, wherein this unit is characterized in that it comprises an impact separator that comprises solid surfaces that are placed opposite the transfer pipe, wherein said impact separator is at least 50% opaque when viewed from said transfer pipe that is located upstream, and wherein the impact separator is placed inside the cone in such a way that there is free passage of at least 40 mm and preferably at least 80 mm between the periphery of the impact separator and the cone over the bulk of the periphery of the impact separator.
The free passage can be equal to at most half the diameter of the tubular plate.
Input cone is defined as a transition zone with a section that expands significantly and with a shape that is generally at least partially tapered, wherein said zone is connected upstream to the pipe for transfer of cracked gases that come from the pyrolysis pipes and downstream to the tubular plate of the quenching exchanger. Said cone can also be flared xe2x80x9ctrumpetlikexe2x80x9d or be open-ended in shape.
Preferably, the impact separator will be at least 70% or even 100% opaque, viewed from the upstream transfer pipe.
An impact separator that is at least 70% opaque is defined as an impact separator for which at least 70% of the stream lines of the transfer pipe when said lines are extended parallel to the axis of the cone meet the impact separator.
In other words, the projected surface area of the various elements of the impact separator, over the end section of the transfer pipe, represents at least 70% of this section. (The section of the pipe is the surface area that is delimited by the circle that corresponds to the inside diameter of the pipe, just. upstream from the cone, wherein the surface area is projected parallel to the axis of the cone.)
The gas passages can be non-communicating or communicating, for example at ends of solid surfaces that constitute the impact separator, as it will be described below.
According to a characteristic variant, the unit is characterized in that the impact separator comprises at least one row of approximately parallel bars. These parallel bars can have, in particular, a circular section, a circular section with cut sides, a square section, or a rectangular section.
The bars can be approximately perpendicular to the axis of the exchanger according to another embodiment.
According to another embodiment, the axis of the bars can have an angle that differs by 90 degrees from that of the axis of the exchanger. These bars can have, for example, a V-shape.
According to another characteristic variant, the impact separator comprises at least two rows of parallel bars that are arranged according to at least two levels, wherein the bars are separated by empty spaces and are offset such that the empty spaces at one of the levels are opposite the bars that are arranged at the other level.
According to a preferred embodiment, the bars are attached to a bracket that is perpendicular to these bars and is arranged approximately in their center.
Finally, according to a characteristic design variant, the input cone comprises a metal outside wall and a metal inside wall that is preferably welded to the outside wall, wherein the space between these two walls is filled by a refractory packing that is a relatively poor heat conductor and is made of, for example, refractory concrete, wherein said impact separator is attached to the inside wall by at least two attachment lugs.